A liquid rubber having amine structure on both ends of the molecule (hereinafter referred to as an “amine terminated rubber”) has been known in the art (see, for example, B. F. Goodrich Chemical Company, Technical Data, A. J. Kinloch, S. J. Shaw, D. A. Tod and D. L. Hunston, Polymer, 24, 1341, (1983) and A. J. Kinloch, S. J. Shaw and D. L. Hunston, Polymer, 24, 1355 (1983)). These amine terminated rubber has a rubber backbone and a reactive amine structure on both ends of the molecule, and therefore, these rubbers have been used for the purpose of imparting toughness (the property which enables bending without fracture) to a thermosetting resin. For example, epoxy resin is used for various applications owing to its excellent dynamic properties (for example, modulus) and heat resistance. However, epoxy resin is insufficient in the toughness required in some particular applications, and the amine terminated rubber is used in such applications.
However, when the amine terminated rubber that had been known in the art was used in combination with a thermosetting resin such as epoxy resin, the resulting composition exhibited insufficient long term stability since the amine terminated rubber had low molecular weight, and the amine terminated rubber was highly reactive with a group such as epoxy group.